This invention relates to radiological well logging methods and apparatus for investigating the characteristics of subsurface earth formations traversed by a borehole, and more particularly, to methods and apparatus for measuring the porosity of earth formations in the vicinity of a well borehole by means of pulsed neutron well logging techniques.
In the search for hydrocarbons beneath the earth's crust one of the parameters which must be known about an earth formation before evaluating its commercial potential is the fractional volume of fluid filled pore space, or porosity, present around the rock grains comprising the earth formation. Several techniques have been developed in the prior art to measure earth formation porosity in a borehole environment. One such technique employs a gamma ray source and a single, or multiple, detectors to measure the electron density of the earth formations by gamma ray scattering. This leads to an inferential measurement of the porosity of the formations. Another technique employs an acoustic transmitter and one or more acoustic receivers. The velocity of sound transmission through the formation from the acoustic transmitter to the receivers is then measured and this quantity can be related to the porosity since sound travels faster in less porous rocks than in fluid filled pore spaces in the earth formations.
A third commercial technique which has been employed in the prior art to measure the porosity of earth formations employs a neutron source and either a neutron or gamma ray detector sensitive to low energy, or thermalized, neutron density. Hydrogen is the principal agent responsible for slowing down neutrons emitted into an earth formation. Therefore, in a formation containing a larger amount of hydrogen than is present in low porosity formations the neutron distribution is more rapidly slowed down and is contained in the area of the formation near the source. Hence, the counting rates in remote thermal neutron sensitive detectors located several inches or more from the source will be suppressed. In lower porosity formations which contain little hydrogen, the source neutrons are able to penetrate farther. Hence, the counting rates in the detector or detectors are increased. This behavior may be directly quantified into a measurement of the porosity via well established procedures.
All of these commercially employed methods have generally not proven to be as accurate as desirable due to diameter irregularities of the borehole wall, variation of the properties of different borehole fluids, the irregular cement annulus surrounding the casing in a cased well borehole, and the properties of different types of steel casings and formation lithologies which surround the borehole. For example, the thermal neutron distribution surrounding a source and detector pair sonde as proposed in the prior art can be affected by the chlorine content of the borehole fluid. Similarly, lithological properties of the earth formations in the vicinity of the borehole, such as the boron content of these formations, can affect the measurement of thermal neutron populations. The present invention however, rather than relying on a measurement of the thermal neutron population comprises a neutron measurement of the formation porosity which utilizes a measure of the epithermal neutron population at one detector and the background corrected fast neutron population at a second detector spaced approximately the same distance from a pulsed neutron source. Special detectors and other means are utilized in the present invention to effectively discriminate against the detection of thermal neutrons or their resultant capture gamma rays as proposed by prior art thermal neutron population measurement techniques.
Thus, it is an object of the present invention to provide an improved method and apparatus for measuring the porosity of earth formations in situ in a well borehole using pulsed neutron source techniques.
Another object of the present invention is to provide an improved technique for measuring the porosity of earth formations in the vicinity of a well borehole by combining measurements of the epithermal neutron population and the fast neutron population using pulsed neutron source techniques.